This invention relates to novel composites useful for controlled generation of heat by absorption of microwave energy.
Food preparation and cooking by means of microwave energy has, in recent years, become widely practiced as convenient and energy efficient. Along with the growth in the use of microwave cooking has been a growth in the sale and use of foods specially packaged for microwave cooking. Such special microwaveable packages attempt to alleviate some of the problems inherent in microwave cooking, for example, lack of browning or crispening of the surface of a cooked food item or uneven cooking due to development of "hot spots" in the food item. Examples of packaging materials developed for use in microwave cooking are those disclosed in U.S. Pat. Nos. 4,518,651, 4,267,420, 4,434,197, 4,190,757, 4,706,108, UK Patent Application No. 2,046,060A and European Patent Application Publication No. 63,108.
U.S. 4,518,651 to Wolfe discloses composite materials exhibiting controlled absorption of microwave energy based on the presence of electrically conductive particles such as particulate carbon in a polymeric matrix bound to a porous substrate. The resulting composite materials are to have a surface resistivity of 100 to 1000 ohms per square. Wolfe teaches that it is critical that at least some of the polymeric matrix be beneath the surface of the substrate, be substantially free of electrically conductive particles and be intermingled with the substrate. This is achieved by a lamination process at certain temperatures and pressures.
U.S. 4,267,420 to Brastad discloses a packaging material which is a plastic film or other dielectric substrate having a thin semiconducting coating. The semiconducting coating generally has a surface resistance of 1 to 300 ohms per square, and the preferred coating is evaporated aluminum. Similar materials, i.e., films with a continuous layer of electrically conductive material deposited thereon, are also disclosed in UK Patent Application 2,046,060A.
U.S. 4,434,197 to Petriello et al. discloses a multi-layer structure having five layers including outside layers of polytetrafluoroethylene, two intermediate layers of pigmented polytetrafluoroethylene and a central layer of polytetrafluoroethylene containing an energy absorber. The energy absorber can be a material such as colloidal graphite, ferric oxide and carbon and should have a particle size such that it will uniformly disperse with particles of polytetrafluoroethylene to form a co-dispersion.
U.S. 4,190,757 to Turpin et al. discloses a microwaveable package composed of a non-lossy dielectric sheet material defining a container body and a lossy microwave absorptive heating body connected thereto, the heating body possibly comprising a multiplicity of particles of microwave absorptive material of different particle sizes and a binder bonding those particles together. Absorptive materials include zinc oxide, germanium oxide, iron oxide, alloys such as one of manganese, aluminum and copper, oxides, carbon and graphite. The binders for these materials are ceramic type materials such as cement, plaster of paris or sodium silicate, and the resulting materials are therefore not flexible. The package also requires a shield, for example, a metal foil sheet adapted to reduce by a controlled amount the direct transmission of microwave energy into the food product. A somewhat similar disclosure is found in U.S. 4,266,108 to Anderson et al. This patent also discloses a microwave heating device comprising a microwave reflective member positioned adjacent to a magnetic microwave absorbing material.
European Patent Application Publication No. 63,108 discloses a packaging material such that at least a region of one side thereof is provided with a coating comprising heat reflecting particles in a predetermined pattern, in for instance flake or particle shape. The heat reflecting particles preferably consist of metal particles of aluminum or another food-stuff inert metal and are preferably included within a layer of polyester, polymethylpentene or another material having corresponding heat resistance characteristics. The content of heat reflecting particles amounts to 0.01-1% by weight of the surface weight of the coating, and the heat resistant layer has a surface weight of 15 to 30 grams per square meter.
Despite the many developments to date in the field of microwaveable packaging, certain needs still exist. Many existing materials function in one way or another to convert a portion of the microwave energy into heat, but the materials offer little control to the packager in terms of how much heat is generated and how quickly. For example, some of the materials tend to heat uncontrollably in a microwave oven, leading to charring or even arcing, ignition and burning of the packaging material. Other available materials are simply not capable of generating enough heat quickly enough to be of use in certain applications (e.g., providing fast heat-up and high bag temperatures to provide efficient popping of popcorn in a microwave oven). And many of the available materials are simply not suitable for the mass disposable-packaging market because they are simply too expensive to produce.